Twine - Twine is an open-source tool for telling interactive, nonlinear stories.
Ink by Inkle - ink is a popular open source scripting language for branching stories, designed for writers
Arcweave - Arcweave is the first browser based cloud professional tool for game and narrative design.
Quest - Quest lets you create sophisticated text-based games, without having to program.
Dialogue Designer - An easy to use but powerful tool for creating branching dialogues for games.
Squiffy - Squiffy is a tool for creating multiple choice interactive fiction.